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## Main Question or Discussion Point

Hi guys,

I've been asked to check somebody's relativity logic and i'd love to get s second opinion as it's easy to slip up.

There is a train, with an observer (A) in the middle, with a device that fires marbles simultaneously up the train and down the train. There is also a detector at each end of the train that has a clock which records when the marble arrives.

There is also an observer (B) on a platform, they also have detectors which independently record the time the marble arrives.

scenario 1

in the case that the train is stationary observer (A) and (B) are at the same point in the axis of the train and therefore have the same frame of reference and the detectors clocks agree

scenario 2

the train is moving, observer (A) sees the marbles reach each of their detectors (on the train) simultaneously. Observer (B) also sees the marbles reach each of their detectors (on the platform) simultaneously.

scenario 3

scenario 2 is repeated but instead of marbles, photons are used, this time observer (A) and (B), detectors show different times and there for conclude that the photons reaching their respective detectors are not simultaneous events.

My first issue is with scenario 2 with the marbles, I do not think the two observers detector clocks would show the same time, and I do not think that replacing marbles with photons would make any difference other than making the relativistic effect more noticeable. it seems like for scenario 2 with marbles they have used classical physics and for scenario 2 with photons they have considered relativity. would anyone agree with my conclusion on this thought experiment? or are they correct and I am missing something? Many thanks for your help :)

I've been asked to check somebody's relativity logic and i'd love to get s second opinion as it's easy to slip up.

There is a train, with an observer (A) in the middle, with a device that fires marbles simultaneously up the train and down the train. There is also a detector at each end of the train that has a clock which records when the marble arrives.

There is also an observer (B) on a platform, they also have detectors which independently record the time the marble arrives.

scenario 1

in the case that the train is stationary observer (A) and (B) are at the same point in the axis of the train and therefore have the same frame of reference and the detectors clocks agree

scenario 2

the train is moving, observer (A) sees the marbles reach each of their detectors (on the train) simultaneously. Observer (B) also sees the marbles reach each of their detectors (on the platform) simultaneously.

scenario 3

scenario 2 is repeated but instead of marbles, photons are used, this time observer (A) and (B), detectors show different times and there for conclude that the photons reaching their respective detectors are not simultaneous events.

My first issue is with scenario 2 with the marbles, I do not think the two observers detector clocks would show the same time, and I do not think that replacing marbles with photons would make any difference other than making the relativistic effect more noticeable. it seems like for scenario 2 with marbles they have used classical physics and for scenario 2 with photons they have considered relativity. would anyone agree with my conclusion on this thought experiment? or are they correct and I am missing something? Many thanks for your help :)